1. Field of the Invention
The present invention relates to a Wireless Sensor Network (WSN). More particularly, the present invention relates to the technology of transmitting/receiving data in a WSN and problems associated with unstable wireless links. Still more particularly, the present invention relates to a Wireless Sensor Network (WSN), and specifically to a method for checking wireless channel connection states and a type of data to be transmitted among devices constructing a WSN, and then controlling data transfer in consideration of the result of the check.
2. Description of the Related Art
A WSN differs from existing networks that have been realized for communication in that the WSN has been embodied for the purpose of collecting remote information. The WSN is typically equipped with a sensor node for processing information collected via a sensor and then transferring the processed information, and a sink node for sending transferred information to the outside. As a network is constructed of a large number of sensor nodes constructs a network, the structure of each sensor node should be simply designed. Also, since a certain sensor node may be arranged in an area to which it is difficult for a person to get access, the sensor nodes should be designed to consume little electric power so that the sensor node may operate for up to several months or several years with the same battery. In addition, the sensor nodes must be designed to have mobility so that each position in which the sensor node has been installed is enabled to be freely moved. Furthermore, even though some sensor nodes existing within the network are subject to being damaged, the WSN must be embodied so as not to be affected by the maintenance of the network.
Meanwhile, IEEE 802.15 Working Group defines the standards for a short-distance wireless network. More particularly, since the IEEE 802.15.4 standard defined by IEEE 802.15 Working Group enables low power short-distance wireless network to be realized, the IEEE 802.15.4 standard is quickly becoming a core technology which is suitable for being applied to a sensor network.
The IEEE 802.15.4 standard defines communication protocol related to the physical layer and the Medium Access Control Layer (hereinafter, referred to as the “MAC Layer”) within a short-distance wireless network. Furthermore, the IEEE 802.15.4 standard discloses data frame transfer between a transmission apparatus and a receiving apparatus. In data frame transfer between a transmission apparatus and a receiving apparatus, the IEEE 802.15.4 provides a transfer process in consideration of three cases, FIGS. 1A to 1C illustrate a transfer process relevant to each case. Hereinafter, with reference to FIGS. 1A to 1C, a description will be made in detail of a transfer process relevant to each case.
(1) In the case of successful transfer of a data frame and an Acknowledgement (ACK) frame:
With reference to FIG. 1A, a transmission apparatus MAC layer 12 is requested to transmit a data frame from a transmission apparatus network layer 11 (step S100), and requests a transmission apparatus physical layer 13 to transmit a data frame (step S110). Then, the transmission apparatus physical layer 13 transmits a data frame to a receiving apparatus 2023 (step S120), and informs the transmission apparatus MAC layer 12 that the data frame has been transmitted (step S130). Accordingly, the transmission apparatus MAC layer 12 enables a timer to operate, and waits for receiving an ACK frame for a prescribed time interval, e.g., a period of time during which receiving the ACK frame from a receiving apparatus 20 is expected (step S140).
Meanwhile, if a receiving apparatus physical layer 23 receives the data frame, the receiving apparatus physical layer 23 informs a receiving apparatus MAC layer 22 that the data frame has been received (step S200). Then, the receiving apparatus MAC layer 22 delivers the received data frame to a receiving apparatus network layer 21 (step S210), and requests the receiving apparatus physical layer 23 to transmit the ACK frame to the transmission apparatus MAC layer 12 (step S220). In addition, the receiving apparatus physical layer 23 transmits the ACK frame to a transmission apparatus physical layer 10 (step S230), and informs the receiving apparatus MAC layer 22 that the ACK frame has been received (step S240).
Still referring to FIG. 1A, the transmission apparatus physical layer 13 delivers the ACK frame received from the receiving apparatus physical layer 23 to the transmission apparatus MAC layer 12 (step S300). The transmission apparatus MAC layer 12 enables a timer to be terminated, and informs the transmission apparatus network layer 11 that the transfer of the data frame has been completed (step S310).
(2) In the case of unsuccessful transfer of a data frame:
With reference to FIG. 1B, in the case of unsuccessful transfer of a data frame, by performing steps S100 to S140 as in the above-described case (i.e., in the case of successful transfer of a data frame), a data transfer is requested, and waiting for the reception of an ACK frame is implemented.
However, a wireless link state is often unstable between the transmission apparatus and the receiving apparatus, and therefore, a data frame transmitted from the transmission apparatus physical layer 13 cannot be delivered up to the receiving apparatus physical layer 23. Accordingly, the transmission apparatus physical layer 13 cannot receive the ACK frame from the receiving apparatus 20. After all, the transmission apparatus MAC layer 12 fails to receive the ACK frame until a timer is terminated, and repeatedly performs steps S110 to S140.
The transmission apparatus MAC layer 12 repeats this process up to three times, and if the transmission apparatus MAC layer 12 cannot receive a special ACK frame, it does not attempt to transmit the data frame again for a fourth time, but informs the transmission apparatus network layer 11 that the transfer of the data frame has failed (step S350).
(3) In a case where the transfer of a data frame has been successful but the transfer of an ACK frame fails:
With reference to FIG. 1C, in a case where the transfer of an ACK frame has failed, as in the above-described case (i.e., in the case of successful transfer of a data frame), the transmission apparatus 10 performs steps S100 to S140 to request data transfer, and waits for receiving an ACK frame. In addition, the receiving apparatus 20 delivers a data frame received through steps S200 to S240, to the receiving apparatus network layer 21, and transmits the ACK frame to the transmission apparatus 10.
However, since a wireless link state between the transmission apparatus and the receiving apparatus is unstable, the ACK frame transmitted from the receiving apparatus physical layer 23 cannot be delivered up to the transmission apparatus 10.
Finally, the transmission apparatus MAC layer 12 fails to receive the ACK frame until the timer is terminated, and repeatedly performs steps S110 to S140. The transmission apparatus MAC layer 12 repeats this process up to three times, and if the transmission apparatus MAC layer 12 cannot receive a special ACK frame, it does not attempt to transmit the data frame for a fourth time, but finally informs the transmission apparatus network layer 11 that the transfer of the data frame has failed (step S350).
Furthermore, Zigbee Union has tried the standardization of protocols of upper layers (the network layer and an application layer) which have not been suggested in the Zigbee protocol based on the IEEE 802.15.4 standard, i.e. the IEEE 802.15.4 standard. However, even in the Zigbee protocol, any plan for overcoming the above problem (i.e., a problem that the transfer of data is not implemented due to coupling errors among nodes) raised in the IEEE 802.15.4 standard has not been considered, either, and a solution to the above problem has not been suggested.
In order to embody a WSN, the reliability of each data frame transmitted among nodes must be secured. However, when the WSN is realized based on a protocol suggested in the IEEE 802.15.4 standard, or the Zigbee protocol, a wireless link state between specific nodes is unstable, and then, an error in data transfer can be caused repeatedly. Namely, a problem that the reliability of a data frame transmitted through unstable wireless link is raised. However, despite the occurrence of this problem, data has been transmitted uniformly by applying the same conditions without considering a state of each wireless link at all. Hence, there is a need in the art to solve at least the above-identified problems in the art.